An object traveling faster than sound creates a shock wave that one typically hears as a boom or sharp report. The sonic boom from a supersonic jet and the cracking sound that is heard from a whip are two examples. When a towel is snapped, does the tip exceed the speed of sound?
This question was answered by three high-school students using a high-speed video-imaging technique that captured the tip of the towel as it was flipping over.
Four images of the towel's tip were captured by four flash units, each with a different color filter. The flashes of light, controlled by a computer, were only 1/2500 second apart. The tip traveled farthest between the 2nd and 3rd images (green and yellow) as it flipped from right to left. The distance traveled was determined from an overlay of a distance scale. The average speed was found to be 360 m/s, slightly greater than the speed of sound!
The photograph was taken by high-school seniors Spence Allen, Nicolas Lee and Elizabeth Smith in the fall of 1992. Their project was sponsored by physics teacher Loren Winters at the North Carolina School of Science and Mathematics (Durham, NC). For more information on this project, see their paper in the September 1993 issue of The Physics Teacher.